New Mexico Geological Society Annual Spring Meeting — Abstracts

The Middle Rio Grande hydrologic basin (MRGB) comprises a linked series of physiographic and structural subbasins between Cochiti and Elephant Butte Reservoirs. It includes valleys of the Rio Grande-Puerco-Jemez fluvial system, and the Santo Domingo, Albuquerque (ABQ), Belen, and Socorro basins of the Neogene Rio Grande rift (RGR). We have long been concerned with the exploitation of river-valley and basin-fill aquifers of the region (Lee, 1907; Bryan, 1909, 1938); but only in the past decade has there been a concerted effort (of hydrologists, geophysicists and geochemists) to develop conceptual models of MRGB hydrogeology that can even begin to meet the quality demands inherent in numerical modelling of the complex ground-water flow system (Hawley et aI, 1995; Kernodle et aI, 1995; McAda, 1996). The maps and cross sections displayed in this poster session illustrate the progress made between 1907 and 1997 in characterizing the hydrogeologic framework of Albuquerque-Rio Rancho Metro-Area segment of the MRGB.

Kirk Bryan (1938) developed the first comprehensive model of RGR basin hydrogeology. His concept of a Rio Grande (structural) "depression" extending from the San Luis Valley in Colorado to beyond EI Paso, Texas, has evolved into our current tectonic model of the RGR (Chapin and Seager, 1975; Kelley, 1977; Keller and Cather 1994). Rift-basin fill deposited prior to Plio-Pleistocene river valley incision comprised the bulk of Bryan's (J938) Santa Fe "formation", which was subsequently raised to group status (Spiegel, 1961). Bryan and several students (McCann, Denny, Wright, and Stearns) showed that exposed Santa Fe deposits in the ABQ Basin could be informally subdivided into lower "Gray", middle "Red", and upper "Buff" members with both Iitho- and hydro-stratigraphic significance. Structural offset of these units along major boundary faults allowed Bryan and McCann (1937, p. 801) "to speculate on the amount of deoression of the basin as a result of this faulting."

Bryan (1938) defined three of the four major lithofacies assemblages that form the basic hydrostratigraphic components of basin-fill aquifer systems throughout the Basin and Range province: 1) Coarse-grained, basin-margin piedmont and related alluvial-slope deposits; 2) Fine-grained, closed-basin lake, playa, and alluvial-flat sediments; and 3) Mixed composition basin-floor deposits of throughgoing streams. Other workers in the RGR also recognized that thick eolian sand deposits locally form a fourth major basin fill component. Finally, Bryan (1938) was among the first to characterize the complex interaction between ground- and surface-water flow systems, precipitation, and evapotransperation in the context of "completely and incompletely enclosed ground-water basins", as well as the above-listed lithofacies assemblages.

Surface and subsurface information acquired over the past 60 years from well drilling, geophysical surveys, geological mapping, petrologic and geochemical analyses, and hydrologic studies give us a much clearer view of the structure, stratigraphy, and lithofacies composition to depths of as much as 1 Km in many RGR basins, but the basic validity of Bryan's (1938) model still holds. General mapping of hydrogeologic units in ABQ Basin area was completed in the early sixties (Bjorklund and Maxwell, 1961; Titus, 1963), with Kelley (1977) providing the first synoptic surface and subsurface view of basin geology (map scale 1'190,000). Prior to the ongoing State/Federal geologic mapping program, however, detailed maps and cross sections (> 1:50,000) depicting basin-and valley-fill stratigraphic and lithofacies characteristics were completed in only a few areas (e.g. Lambert, 1968; Machette, 1978; Lozinsky, 1988; and Tedford, 1991). Incorporation into the MRGB hydrogeologic framework model of recent information from deep drilling and geophysical surveys for hydrocarbon, geothermal and ground-water resource evaluation represents a major advance in our understanding of subsurface conditions.